I didn't see anyone mention this here, so I will . . .

These three structures are called "obelisks." Most people think of them as being inspired from Egyptian architecture. Yes, the Egyptians used them. But the word "obelisk" is derived from the deity Baal from the region of Canaan.

Obelisk actually means "shaft of Baal."

Worshippers of Baal used to sacrifice children to this "God."

At times of crisis, Baal's followers sacrificed their children, apparently the firstborn of the community, to gain personal prosperity.

You have to use discernment in everything today as even this link is scrubbing the act of sacrifice as being "In times of crisis" as a way of rationalizing this behavior.

IMO, the Washington Monument being dark is symbolic that Baal no longer has power there.